Electronic image scanners convert an optical image into an electronic form suitable for storage, transmission or printing. In a typical image scanner, light from an image is focused onto line-arrays of photosensors for scanning one line at a time. A two dimensional image is scanned by providing relative movement between the photosensor line-arrays and the original image. In general, a color scanner measures the intensity of at least three relatively narrow bands of wavelengths of visible light, for example, bands of red, green and blue.
For image scanners, the digitized image may be degraded by the presence of artifacts on the surface of the object being scanned, such as dust and fingerprints, or defects in the surface of the object being scanned, such as scratches, folds, or textured surfaces. Multiple methods have been disclosed for detecting defects on transparent media. See, for example, U.S. Pat. No. 5,266,805, U.S. Pat. No. 5,969,372, and EP 0 950 316 A1. Some of the methods in the referenced patent documents utilize the fact that the dyes in transparent color film are essentially transparent to infrared light, whereas dust and scratches are relatively opaque. Other disclosed methods utilize dark field imaging, in which the light reaching the photosensors is reflected or diffracted by defects instead of the film.
Scanners for opaque media are configured differently than scanners for transmissive media, and different detection methods are needed. Commonly assigned U.S. patent application Ser. No. 09/629,495, filed Jul. 3, 2000 discloses defect detection, in an opaque media scanner, having multiple spaced-apart line-arrays of photosensors, where surface defects cast shadows, and the length of the shadows, as seen by each line-array of photosensors, varies among the different line-arrays of photosensors.
Reflective document scanners and copiers commonly provide a fixed-position calibration target, along a scanline dimension. In a flat bed scanner with a motionless document being scanned, the calibration target is typically beneath a glass platen in a relatively dust free environment. The calibration target is used to compensate, before scanning, for variation in sensitivity of individual photosensors, and for variation in light intensity along the length of the scanline. The process is called Photo-Response Non-Uniformity (PRNU) calibration. See, for example, U.S. Pat. No. 5,285,293. Since the calibration target is presumably uniform, any pixel to pixel intensity variation can be attributed to sensor sensitivity, light source variation, or other system uniformity. A correction factor (gain and/or offset) is calculated and applied to subsequent scans. Just in case there is a surface defect on the calibration target, it is known in commercially available scanners to accumulate data from many scanlines (the photosensors are moved relative to the calibration target) during PRNU calibration, and to average the data on a photosensor-by-photosensor basis. It is also known to discard extreme data points before averaging. For example, given ten intensity measurements for one photosensor, it is known to discard the lowest and highest intensity values, and then average the remaining eight values. This helps eliminate the effects of surface defects on the calibration target during PRNU calibration.
Of particular concern in the present application is scanners in which a document moves past a stationary photosensor array, for example, scroll-feed scanners, and flat-bed scanners with automatic document feeders. Scroll-feed scanners and scanners with automatic document feeders have several unique problems regarding detection and correction of surface defects. In a scroll-feed scanner, the document may be displaced from a platen, so the calibration target is typically behind the document being scanned to properly measure the light at the document. This in turn causes three potential problems. First, the calibration target is much more susceptible to debris introduced by documents, particularly paper debris. Second, the photosensor array is typically stationary, so the technique of averaging multiple scanlines to eliminate the effects of surface defects during PRNU calibration is not applicable. Third, if the calibration target is behind the document being scanned, and if there is debris on the calibration target, then the PRNU calibration compensates for the debris, but subsequent document scanning hides the debris on the calibration target so that the PRNU gain or offset is inappropriate. The result is a streak in the digitized image. Finally, with moving documents, it is common for debris to become temporarily trapped between the document and a glass platen, and then later the debris may be dislodged. Again, the result is a streak in the digitized image.
There is a need for surface defect detection for the unique situations presented by moving documents: (1) debris on the calibration target with a stationary photosensor array, and (2) temporary debris on a platen during scanning.